In recent years, research and development of projection-type imaging devices that project an enlarged image on a screen via a projection lens, using, e.g., a halogen lamp or high-pressure mercury lamp that emits incoherent light as, e.g., a liquid-crystal light valve, have actively been conducted. However, projection-type imaging devices and image projection devices related to the present invention have a problem in consuming a large amount of power and having only a weak brightness because they use spontaneously-emitted incoherent light. Also, the incoherent light sources have a problem in that it is difficult to provide a display having a broad chromaticity range because of the broad wavelength bands of the respective light sources for red, green and blue. Research and development of image display devices using laser light that produces coherent light for light sources have been conducted as a method for providing a display that has low power consumption, strong brightness and a broad chromaticity range.
Japanese Patent Laid-Open No. 11-305710 discloses an image projection device that uses laser light as coherent light sources. FIG. 1A is a diagram for describing an image projection device related to the present invention, the image projection device using coherent light sources. In the image projection device disclosed in Japanese Patent Laid-Open No. 11-305710, as illustrated in FIG. 1A, laser light beams emitted from red (R) pulse laser 1, green (G) pulse laser 2 and blue (B) pulse laser 3 are combined by dichroic prism 4. The combined light is subjected to in-plane light amount uniformization by means of integrator 5, and then illuminates a light modulation element (light valve) 6. Then, an image in the light modulation element (light valve) is projected on screen 8 by means of projection lens 7, thereby a color image is displayed.
However, in the case of an imaging system using a projection lens in an image projection device using laser light for coherent light sources, the imaging system comes into focus only within a focal depth of the projection lens. Thus, it is necessary for a user to perform focusing according to the screen position. This causes a problem in reducing the convenience of using a portable projection-type image display device.
Projection-type display devices that scans laser beams, which travel in a straight line, to project and display an image are disclosed (for example, in Japanese Patent Laid-Open No. 2003-21800) as a method for eliminating the need for a user to perform focusing in a certain projection range. FIG. 1B is a diagram for describing an example of a projection-type display device related to the present invention, the projection-type display device scanning a laser beam to project and display an image. In the projection-type display device disclosed in Japanese Patent Laid-Open No. 2003-21800, as illustrated in FIG. 1B, laser beams emitted from red, green and blue light sources 11, 12 and 13, respectively, are combined by color-combining element 14 the combined light is then collimated by collimator lens 15 so that the beam waist of the laser beam comes to the vicinity of projection surface 18 as a pixel 19, and then, is two-dimensionally scanned via optical scanning elements 16 and 17, which perform horizontal and vertical optical scanning, thereby a color image is displayed.